Such a disconnectable mooring system is disclosed in US patent application US2007/155259. The known system includes a buoy that is provided with a conical outer casing and a corresponding conical cavity or receptacle on the vessel's turret structure, which cavity has a cone shape corresponding to the conical outer casing of the buoy member. The turret structure includes a turntable carrying conduits to be connected to the risers, wherein the turntable is supported on a bearing assembly in a manner allowing rotation with respect to the turret structure to align the conduits with the risers on the buoy only after the buoy is received and locked in the cavity of the turret structure. In this publication it is shown that only a main turret upper roller ball bearing assembly supports the turntable; this assembly includes three mutually movable parts that are directly interconnected to each other. In fact, this upper turret bearing assembly consists of 2 roller ball bearings that are directly placed on top of each other and interconnected via one common inner bearing housing member. This upper bearing assembly has therefore become a very critical and essential part of a weathervaning system. A disadvantage of this combined and interconnected roller ball bearing assembly is that if one or more roller balls fails, the complete assemble has to be changed out, meaning that the turret system cannot function anymore as a weathervaning system. This change out cannot be done offshore.
The known combined roller bearing system, due to the fabrication limitations, is limited to about only 8 meters, so that it not suitable for large disconnectable turret-buoy systems with for example 20 or more risers connected to the buoy.
Another patent publication that describes a disconnectable mooring system that is provided with two separate bearing systems, one of which is used only for rotating a turntable in order to align the manifold pipe ends with the riser ends of a connected buoy, is U.S. Pat. No. 5,651,708. This patent shows a disconnectable buoy that is provided with a bearing system that stays with the boy when disconnected. The buoy is rotatable connected to the moonpool of a vessel under the waterline without the use of a turret. An additional upper bearing system is disclosed at deck level, which supports a turntable with manifold, so that after the buoy is connected directly to the moonpool of the vessel, the turntable can be aligned with the risers of the connected buoy. The turntable is supported by the bearing system, so that even during production when hydrocarbons are received through the flexible piping connecting the manifold and the buoy, the turntable can be rotated at all times and be aligned with the buoy. When the twisting angle in the flexible piping between the buoy and the turntable is exceeded, the turntable is rotated by means of a connected motor driven pinion to a new position neutralizing the twisting. This system is therefore not advantageous for disconnectable turret-buoys systems sized to receive numerous of risers, and of course is not possible when using only hard piping.
Another disconnectable mooring system is described in US patent publication U.S. Pat. No. 5,823,131. This patent discloses a disconnectable riser buoy for supporting only risers or riser lines, but with no mooring lines attached to it.
This riser buoy can be docked within a rotatable turret placed in a moonpool of a floating vessel and carries risers that are connected to flow paths, which are removably coupled to vessel product lines at a position above sea level. When the riser buoy is disconnected from the turret, it is maintained at a submerged depth in the sea by a weight attached to a buoy anchor leg that can be lowered down to the sea floor or raised within the turret. The turret is directly anchored to the sea floor via multiple mooring lines that are connected to the lower turret. When the riser buoy is released, the weight connected to the riser buoy, once resting on the sea floor, will moor the riser buoy and as such limit the excursions of the risers within acceptable limits. Further, as the mooring legs are directly connected to the turret, the riser buoy has only sufficient buoyancy to support the risers.
Another major aspect of this concept is that in order to dock the riser buoy, a retrieval line is pulled upwardly via a winch until the weight contacts the buoy. Then, buoy and weight are hooked up together, the weight being in contact with the bottom of the riser buoy and both riser buoy and weight are placed within the moonpool of the vessel. The main purpose of this system is to allow for hook-up of a pre-installed riser buoy before installation of the vessel and prior to connecting the mooring lines to the turret takes. The known mooring system does not function as a quick disconnectable system that is suitable to be used in cyclone areas or ice infested waters as the mooring legs stays connected to the turret. Also hook-up of both the riser supporting buoy and the weight together is only possible for relatively small buoys and weights and not for large buoys with large connected weights, as this would require a winch capacity exceeding the capacity of winches available in the field and involving the danger of creating large snap-loads in the hauling-in line that connects the buoy and the winch. This results in large winches that are designed to withstand such snatch loads.
In these known systems the capability to reconnect a buoy to a turret is mainly limited by the sea state and winch capacity. When the buoy is brought upwards to the turret for reconnection purposes, the heave motions of the buoy are coupled to those of the vessel when the buoy approaches its connect position. If the sea states are too large, snatch loads and buoy acceleration forces are exerted on the connection lines that exceed the strength of available reconnection lines. This is especially the case for large size buoys, for instance carrying 20 risers or more.
It is therefore an object of the present invention to provide a disconnectable turret-mooring buoy design having an increased reconnection capability even in severe sea states of for example up to 6 m significant wave height.
It is a further object of the present invention to provide a quick disconnectable and easy connectable mooring buoy system for a large numbers of risers and mooring legs, in which snatch loads on the pull-in line are reduced.
It is a further object of the present invention to provide a disconnectable mooring buoy system, which can operate with winches or reconnection chain jacks of reduced size.
The system according to the invention should readily connect and disconnect even in very severe environmental conditions to a floating vessel, for example a floating production unit (FPU or FPSO), using a conventional pull-in line, such as a chain. The buoy should provide accommodation for a large number of risers, for example at least 20 risers and 10 umbilicals, in a turret to which the mooring buoy can be connected. The system according to the present invention should ensure a high availably of the system under all weather conditions and minimize the down time before reconnection even considering the constant severity of the environment.